1. Field of the Technology
The present application relates generally to mobile communication devices adapted to operate for communications in two or more different types of wireless communication networks, such as wireless wide area networks (WWANs) and wireless local area networks (WLANs), and more particularly to techniques for permitting the switching of communications between such networks.
2. Description of the Related Art
When a mobile communication device is designed to communicate through more than one physical radio domain, the mobile device must autonomously select one of the domains through which to communicate or attach. This is the case, for example, for mobile devices that support both wireless local area network (WLAN) and cellular radio access technologies, such as Global System for Mobile communications (GSM)/General Packet Radio Service (GPRS) or Universal Mobile Telecommunication Systems (UMTS). In such environments, the cellular radio fading channel environment exhibits very different characteristics than a WLAN fading channel environment. The cellular radio environment is optimized for wide area coverage, while the WLAN environment is optimized for very localized coverage.
For a GSM mobile device, cell selection rules outlined in 3GPP specification documents govern cell selection techniques. Specifically, 3GPP specifications teach how a GSM/UMTS mobile device selects different serving cells in idle and packet transfer modes of operation. See 3GPP TS 45.008, “Technical Specification; 3rd Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; Radio subsystem link control,” (3rd Generation Partnership Project (3GPP) Technical Specification); specifically clause 6.4 “Criteria for cell selection and reselection” and 6.6.2 “Path loss criteria and timings for cell reselection.” The method taught in the 3GPP specification specifies a number of parameters, including a minimum RSSI required to access a candidate cell and a minimum amount of time the candidate cell should be the strongest in the list of monitored cells. A candidate cell may be selected by the mobile device for two different reasons: (1) the path loss criterion (C1) falls below zero for five (5) seconds (i.e. essentially no signal exists at the receiver), or (2) the calculated value of radio signal strength indicator (RSSI) criterion (C2) is greater than the C2 of serving cell for more than five (5) seconds.
Now consider the case where the mobile device is adapted to operate with multiple radio access technologies (RAT) and must autonomously select between a wide area cellular network (e.g. GSM/GPRS) and a WLAN. For example, the mobile device may initially operate in the GSM/GPRS network and be moved near a window or door that permits propagation of a very strong WLAN signal. If cell selection rules similar to that outlined in the 3GPP specification were utilized, the mobile device would likely select WLAN mode if (1) its RSSI exceeds a certain threshold level and (2) the RSSI of the candidate cell is stronger than the current serving cell for greater than some length of time (e.g. five or ten seconds).
However, it may be asked whether such technique would be suitable given the intention of the entire system. The answer would ultimately depend on whether the mobile device is likely to remain in the WLAN domain for some reasonable period of time. In the case above, it is unlikely that incurring the substantial overhead of selecting the WLAN would be desirable as the mobile device would likely reselect back to the cellular system almost immediately. It is believed that the intention of a suitable selection algorithm should be to select WLAN mode if the mobile device is likely to remain within its coverage area for some reasonable period of time. An important consideration for heterogeneous radio system selection is therefore not simply how fast the mobile device is physically traveling, but perhaps how often it has entered and exited cells and/or system boundaries.